The present invention relates to a device for removing a radioactive deposit from a fuel assembly in a nuclear plant which comprises a container, arranged to accommodate the fuel assembly, first means arranged to feed a fluid into and through the container, said fluid being provided to release radioactive deposit from the fuel assembly by means of abrasion and to transport radioactive deposited material out of the container, and second means being arranged to receive the radioactive material transported out of the container by the fluid.
One of the reasons a device such as the present invention is needed in nuclear plants is the deposition of radioactive deposits onto surfaces of the fuel assemblys. Co-60 contributes to a great deal of the radiation burden from these deposits. Furthermore, Co-60 has a half life of 5.3 years, which means that it will not suffice to let the plant stand idle for only a short period in order to permit the radioactive radiation to decay.
The water in reactor water circuits and feed water circuits of the plant cause a precipitation of small amounts of material from the different components with which it is in contact. A great deal of these components are made of stainless steel from which iron, nickel and small amounts of cobalt are precipitated as ions and particles. In older nuclear plants, some components in the reactor and feed water circuit, such as valves, comprise cobalt-rich materials, increasing the amount of precipitated cobalt. The metals precipitated in the reactor water and feed water are deposited onto surfaces in the circuit as oxide, such as so called crud. The crud deposits exist as different types of metal oxides which are subjected to a strong neutron irradiation when, for instance, they are located on cladding tubes for nuclear fuel. Thereby, the metal atoms in crud deposits are transformed into other isotopes, a part of which are radioactive. Particles fall off from and ions are precipitated from the radioactive crud deposition and are thus transferred to the water. Thereby, the particles and the ions are transported with the reactor water to parts located outside the very reactor core and, thereby, distribute the radioactivity to those parts. The radioactive particles and the ions are deposited onto surfaces outside the core as a secondary crud deposit. Accordingly, a crud deposit is also formed outside the core, contributing to the radiation burden of exposed personnel, above all gamma irradiation (xcex3-irradiation), for instance during maintenance and repair work.
Accordingly, it is desirable to avoid large amounts of crud being present at the fuel assemblies, each of which comprises a plurality of cladding tubes, in the nuclear plant. For instance, this can be done by repeatedly removing from the surfaces of the cladding tubes the crud layers that are formed at the surfaces of the fuel assemblies, that is the cladding tubes, and taking care of those layers. However, with the risk of damaging the surface of the cladding tubes during the removal of such crud layers in view, prior art, as for instance the treating by means of chemicals or mechanical treatment, for example by grinding, is unsuitable. Moreover, it is desirable that the removal of the crud layers can be done relatively quickly, in order to avoid a negative effect upon the efficiency of the plant. It is not necessary to remove all the crud, as a trial of removing firmly bound crud present closest to the surface of the cladding tubes also, most probably, will result in a detrimental effect upon these surfaces. Instead, one should primarily remove loosely bound crud present outside the firmly bound crud layer or layers.
According to the Japanese patent JP 52-12 2793 this problem has been solved by means of a device that comprises a container which is arranged to accommodate a fuel assembly and which is connected to a circuit by means of which a medium comprised by a mixture of liquid and solid particles, for example water and ice, is fed into and through the container. Thereby, the medium will move along the cladding tubes included in the fuel assembly and, thereby, release the loosely bound crud layers present on these cladding tubes by means of abrasion. The medium brings the crud material removed from the fuel assembly with it, and, thereafter, the medium is collected in a filtering device included in the circuit. The solid particles are of such a kind that they melt and form a fluid phase as they pass the hot fuel assembly. Thereby, any extensive need of taking care of such solid particles in the filter device is avoided. In order to obtain an evenly distributed abrasion along the length of the fuel assembly, the circuit system is arranged to interchangeably feed the medium in a first and a second direction respectively to the container. For this purpose, openings are arranged in opposite end walls of the container. However, this device has not found commercial use, probably due to irradiation levels that are too high.
One object of the present invention is to provide a device for removing radioactive deposits from a fuel assembly in a nuclear plant, said device having such a construction that it results in a leakage of radioactive radiation as small as possible. The fuel assembly is to be reused after the treatment.
This object is obtained by means of the device initially defined, said device being characterized in that at least the second means are arranged in a xcex3-radiation-dampening medium.
Thereby, particularly xcex3-radiation from the radioactive material received by the second means is prevented from leaking out into the environment and making it difficult for or preventing personnel from being present in said environment.
According to a preferred embodiment of the inventive device, the second means comprises a filter, arranged to separate the radioactive material released from the fluid. Thereby, the device permits the radioactive material to be collected where it is surrounded by the xcex3-radiation-dampening medium, while, at the same time, the used fluid can be further transported and possibly once again fed by the first means into the container for a continued removal of the radioactive deposition.
According to another preferred embodiment of the inventive device, the xcex3-radiation dampening medium comprises water. Thereby, amongst others, the advantage of using an easily accessible and easy to handle medium is obtained. Amongst others, relatively unproblematic handling of the second means while they are located in the xcex3-radiation-dampening medium is thereby permitted.
According to another preferred embodiment of the inventive device, the container is arranged in the xcex3-radiation-dampening medium. Thereby, xcex3-radiation is effectively prevented from leaking out of the container to the environment. As the xcex3-radiation-dampening medium comprises a liquid such as water, it also has the advantage of accomplishing a cooling of the container surrounding the fuel assembly which most of the time is hot.
According to another preferred embodiment of the inventive device, the second means and the container are arranged in a basin which is filled with the xcex3-radiation-dampening medium. Thereby, the advantage of having the radioactive material removed from the fuel assembly and transported to the second means while being surrounded by the xcex3-radiation-dampening means is obtained, such that the radiation of the radioactive material to the environment is minimized during the transport thereof to the filter where it is separated from the fluid and collected. Moreover, for instance, such a solution is advantageous from a space point of view.
According to another preferred embodiment of the inventive device, the fluid comprises a mixture of a liquid and solid particles, preferably a mixture of water and ice. Thereby, the release of the radioactive material, the deposition, from the fluid assembly will preferably occur by means of abrasion, as the solid particles and the liquid move along the length of the fuel rods. Particularly, when the fluid comprises a mixture of water and ice, the ice will melt during the passage along the fuel rods and will finally leave the container in a liquid state. Thereby, these solid particles are prevented from later being caught by the filter for the reception of the radioactive material. The particles which at least initially are solid may of course also comprise any other suitable and meltable material than ice, for example a polymer, but ice has primarily the advantage of comprising water molecules, enabling it to form a homogenous mixture with the liquid as the latter is comprised by water.
According to another preferred embodiment of the inventive device, the fluid comprises a gas and/or steam, preferably water steam. Thereby, a turbulence of the fluid as it is fed through the container with the fuel rods therein is created, resulting in the solid particles getting a further distance along the fuel assembly before they melt, whereby a more evenly distributed abrasive effect is obtained along the length of the fuel assembly. In other words, a smaller amount of solid particles is required to obtain a result corresponding to the one obtained without gas. Prior art seeks to obtain similar effects by interchangeably feeding the fluid through the container from different directions. By combining the use of gas/steam with such a solution according to prior art, a further improved effect may be obtained.
According to another preferred embodiment of the inventive device, the gas comprises air.
According to another preferred embodiment, the device comprises means arranged to send infrasound towards the fuel assembly, The infrasound causes tensions in the radioactive deposition, facilitating the release of said deposition from the fuel assembly.
According to another embodiment, the device comprises a member arranged to rotate the fuel assembly. Thereby, a uniform flow over the cross-section of the fuel assembly is guaranteed, and regions with substantially different flow conditions are avoided. This in its turn results in an increased abrasive effect.
According to another preferred embodiment, the device comprises means arranged to heat at least a part of the fluid that is fed into the container. These means are preferably arranged to periodically intensively heat the fluid in order to obtain temperature changes in the fluid passing along the fuel assembly. The temperature changes of the fluid result in temperature chocks for the radioactive deposition, resulting in an increased degree of fracture formation in the deposition and a facilitated release of said deposition.
Further advantages and features of the present inventions will be disclosed in the following detailed description and in the rest of the dependent patent claims.